Titanium alloys are well known for their lightweight, high resistance to oxidation or corrosion, as well as the highest specific strength (the strength-to-weight ratio) amid all metals except beryllium. Currently, titanium alloy parts have been produced by ingot metallurgy processes including melting, forming and machining (processes), or by powder metallurgy techniques. The first method is not cost effective but provides high levels of all properties of titanium alloys. The second method is cost effective but cannot completely realize all advantages of titanium alloys due to inferior mechanical properties.
Various powder metallurgy processes have been developed during the last three decades for the fabrication of near-net shape titanium articles with density and mechanical properties acceptable for the intended applications. The use of elemental powder mixtures, controlling the particle size distribution, vacuum sintering, hot isostatic pressing, and special surface finishing are among those new developments. But all of these processes, as well as conventional powder metallurgy techniques, impose certain limitations with respect to the characteristics of the produced titanium alloys.
For example, a method for producing sintered articles from a titanium powder alloy disclosed in JP 06092605, 1998 includes molding a mixture of elemental powders, vacuum sintering, hot isostatic pressing of the alloy in α+β region, and shot pinning to heal surface porosity. The irregular porosity in the interior portion of the sintered articles is the drawback of this method, which degrades the mechanical properties, especially the strength.
The method for producing titanium alloys from elemental powders disclosed in JP 129864, 1990, includes pressing of the powder mixture, vacuum sintering, quenching of the alloy in β-region, and hot pressing at a temperature over 800° C. The oxidation taking place during hot pressing reduces mechanical properties.
The method described in the U.S. Pat. No. 4,432,795 includes grinding particles of light metals to the particle size less than 20 μm, mixing them with particles of titanium based alloys having a particle size larger than 40 μm, and compacting the mixture by molding and sintering at temperatures less than that of a formation of any liquid phase. This method allows the manufacture of the alloy having a density close to the theoretical value but the resulting alloy, contaminated by oxygen, iron, and other impurities, also exhibits reduced mechanical properties preventing its use for critical applications.
The U.S. Pat. No. 4,838,935 describes the use of titanium hydride together with titanium powder in the primary mixture before molding and sintering. The molded article is heated in a hot-press vacuum chamber to a temperature sufficient for the dehydration of TiH2 to remove gases. Then, the article is heated to a temperature of 1350-1500° C. while maintaining the pressure and vacuum. This method cannot completely prevent the oxidation of highly-reactive titanium powders during the second heating, because hydrogen is permanently outgassing from the working chamber. Besides, this method is not suitable for powdered mixtures containing low-melting metal and phases.
A preliminary partial sintering of titanium and titanium hydride powders with elemental powders of alloying metals is disclosed in U.S. Pat. No. 3,950,166 granted to K. Obara et al. The “mother” alloy obtained in such a way is pulverized and remixed with powder metals such as Mo, V, Zr, and Al—V master alloy to achieve the final composition of titanium alloy. This mixture is molded in a predetermined shape and sintered at 1000-1500° C. in a vacuum. The preliminary sintering partially improves distribution of alloying components, but creates oxidation of the “mother” powder during pulverization.
The method of said U.S. Pat. No. 3,950,166 includes two sintering stages (preliminary sintering and final sintering) at constant pressure of vacuum or argon, and pulverization of the preliminary sintered master alloy. Such complex process is necessary because a metallurgical reaction between alloy components is not completed within the first sintering stage. Hydrogen does not participate in the reaction due to permanent outgassing during vacuum sintering or diluting by argon when sintering performed in argon. In order to complete the reaction and obtain uniform composition of required alloy, this method includes additional pulverization of master alloy, adding new portion of the components, and re-sintering. And after such an extremely labor consuming and not effective processing, the final articles have density only 95-98% of the theoretical value. Low density and low strength of the final product is caused by said additional pulverization because each additional pulverization of titanium-containing metals results in additional oxidation and accumulation of micro-structural defects and impurities.
The U.S. Pat. No. 5,441,695 granted to T. Gladden is one of the patents related to sintering of titanium hydride powder. However, this process relates to the manufacture of the material containing Titanium Nitride compound because the final step of the process disclosed by this patent is sintering at 1200° C. in nitrogen atmosphere. The author called this atmosphere as non-reactive, but this may be a questionable determination because nitrogen is reactive atmosphere for titanium powder. The compound of TiN is formed over 800° C., and solubility of nitrogen in solid titanium is about 6 wt. % after heating at 1200° C. [see Vol A.E. Structure and Properties of Binary Metal Systems, v. 1, 1959, p. 145]. Such high level of nitrogen is absolutely inadmissible in any titanium alloys used for structural parts in any industrial applications. But, Titanium Nitride is desirable for decoration purposes due to gold-like brilliant color of this compound. Coloring of titanium was a goal of the U.S. Pat. No. 5,441,695, which is clear from the title and from the Example, wherein “the sintered part has an intense, brilliant appearance” (line 46 in column 4). Thus, this patent is not applicable to the manufacture of any structural titanium alloys including titanium alloys containing aluminum and vanadium due to the formation of aluminum nitrides and vanadium nitrides degrading mechanical properties of the titanium alloys. Moreover, the product manufactured according to this patent is full of oxygen and hydrogen too, as it is shown below.
The U.S. Pat. No. 6,551,371 granted to T. Furita, et al. also discloses the use of titanium hydride, however, there is no titanium hydride neither in claims nor in examples, only dehydrated titanium powder was used everywhere. The particle size ratio mentioned in this patent relates to the particles of titanium boride and particles of elemental alloying metals such as aluminum, zirconium, silicon, etc. The ratio mentioned in the claim 15 relates to particles of TiB2, TiC and Y2O3. These ratios are needed to optimize compaction and to provide uniform distribution of reinforcing particles along of the composite structure. This means that said ratios of particle sizes are not effective for homogenizing chemical composition of the matrix titanium alloy.
Several attempts have been made to improve the density and purity of sintered titanium alloys by using titanium hydride as the raw component, together with other alloying powders, as in JP 07278609, 1995, or JP 06088153, 1994, or U.S. Pat. No. 3,472,705, 1969, or WO 9701409, 1997. All of these methods include vacuum heating and sintering accompanied with permanent outgassing. So, the “cleaning effect” of hydrogen is not used properly, and partial oxidation reoccurs after the removal of hydrogen from the vacuum chamber. Thus, these methods do not provide an effective improvement of mechanical properties of sintered alloys, in spite of the sintering promoted by thermal dissociation of titanium hydride.
Some specialized technologies were offered to manufacture titanium alloys in hydrogen atmosphere in JP 58034102, 1983 and CH 684978, 1995. These methods cannot prevent the contamination of sintered metals as well as the methods mentioned above: after the replacement of a hydrogen-containing atmosphere by an inert gas, the oxidation of reactive powders reoccurs.
All other known processes for making near-net shape titanium alloys from metal powders have the same drawbacks: (a) insufficient purity and low mechanical properties of sintered titanium alloys, (b) irregular porosity and insufficient density of sintered titanium alloys, and (c) low reproduction of mechanical properties that depend on the purity of raw materials.